A bundle of permselective hollow fibers is disclosed, containing a plurality of filaments having a fineness of from 0.05 to 20 denier at a filament-to-hollow fiber ratio of from 2 to 3000. The filaments intermingle with the hollow fibers substantially lengthwise over the length of the hollow fibers, and substantially all of the filaments each engage with at least one of the hollow fibers, thereby placing the hollow fibers into an inseparable bundle. The bundle has not only a high packing ratio of hollow fibers, but also an effective arrangement for efficiently exerting a high performance of each hollow fiber. The bundle can be constructed into a fluid separator, which is useful for dialysis, ultrafiltration, osmosis, reverse osmosis, pressure separation of gas mixtures and the like.